Dielectric properties of material with random off-center defects: Monte Carlo simulation of relaxor ferroelectrics

A Ginzburg-Landau type theory of interaction of randomly distributed local dipoles in a paraelectric crystal is developed. The interaction is caused b y the polarization of the host lattice generated by these dipoles. The obta ined effective Hamiltonian of the dipole-dipole interaction is employed for the Monte Carlo simulation of ferroelectric properties of a system with of f-center dopant ions producing local dipoles. The computer simulation shows that at low dopant ion concentration the paraelectric state transforms int o a macroscopically paraelectric state consisting of randomly oriented pola r clusters. These clusters amplify the effective dipole moment and dramatic ally increase the dielectric constant. The interaction between the clusters results in a spectrum of relaxation time and transition to the relaxor sta te. The real and imaginary parts of the susceptibility of this state are ca lculated. At intermediate dopant concentration, the material undergoes a di ffuse phase transition into a ferroelectric state smeared within a temperat ure range. A further increase in the dopant concentration makes the transit ion sharper and closer to the conventional ferroelectric transition. The re sults obtained are compared with the behavior of the K1-xLixTaO3 relaxor fe rroelectric. (C) 2001 American Institute of Physics.